It is known that welded steel parts can be fabricated from steel blanks that have different compositions and/or thicknesses, which are continuously butt-welded. In one known method of fabrication, these welded blanks are cold worked, for example by cold stamping. According to a second known fabrication method, these welded blanks are heated to a temperature that makes possible the austenitization of the steel followed by hot forming and rapid cooling in a forming die. The invention relates to this second mode of fabrication.
The composition of the steel is selected to make it possible to carry out the heating and hot forming steps and to confer a high mechanical strength, high impact strength as well as good resistance to corrosion to the final welded part. Thanks to their ability to absorb impacts, steel parts of this type have applications in particular in the automobile industry and more particularly for the fabrication of anti-intrusion parts, structural parts or parts that contribute to the safety of automotive vehicles.
Among the steels that exhibit the characteristics required for the applications mentioned above, coated steel sheet as described in publication EP 971044 includes in particular a pre-coating of an aluminum alloy or an aluminum-based alloy. The sheet is coated, for example by hot dip coating, in a bath having silicon and iron in controlled quantities, in addition to aluminum. After the hot forming and cooling, it is possible to obtain a mainly martensite microstructure, and the mechanical tensile strength can exceed 1500 MPa.
A known method for the fabrication of welded steel parts includes procuring at least two steel sheets as described in publication EP971044 to be butt welded to obtain a welded blank, optionally to cut this welded blank, then to heat the welded blank before carrying out a hot forming operation, for example by hot stamping, to confer on the steel part the form required for its application.
One known welding technique is laser beam welding. This technique has advantages in terms of flexibility, quality and productivity compared to other welding techniques such as seam welding or arc welding. However, in the assembly methods that include a melting step, the aluminum-based pre-coating made up of a layer of intermetallic alloy in contact with the steel substrate, topped by a metal alloy layer, is diluted during the welding operation with the steel substrate within the molten zone, which is the zone that is liquefied during the welding operation and solidifies after this welding operation, forming the bond between the two sheets.
Two phenomena can then occur:                the first phenomenon is that an increase in the aluminum content in the molten metal resulting from the dilution of a portion of the pre-coating in this zone leads to the formation of intermetallic compounds. These compounds can be sites for the initiation of cracks when mechanical stress is applied.        the second phenomenon is that aluminum, which is an alphagenic element in solid solution in the molten zone, retards the transformation into austenite in this zone during the heating step preceding the hot stamping. It is therefore no longer possible to obtain a fully tempered structure in the molten zone after the cooling that follows the hot forming, and the welded joint includes ferrite. The molten zone then has a hardness and a mechanical tensile strength less than that of the two adjacent sheets.        
To prevent the first phenomenon described above, publication EP 2007545 describes a method that includes removing the superficial layer of metal alloy on the periphery of the sheets to be subjected to the welding operation, leaving the intermetallic alloy layer. This removal can be carried out by brushing, machining or by the application of a laser beam. In the latter case, the width of the removal zone is defined thanks to the longitudinal movement of a laser beam of a certain width, or even by the oscillation of a laser beam smaller than this width, using the edge of the sheet as a reference point. The intermetallic alloy layer is retained to guarantee satisfactory corrosion resistance and to prevent the phenomena of decarburization and oxidation during the heat treatment that precedes the forming operation.
To prevent the second phenomenon mentioned above, publication WO2013014512 describes a method that includes, in addition to removing the metal layer described above, the elimination of the aluminum present on the cut edge of the sheets before welding, the presence of which, can result from a cutting operation, and to create a welded joint with a filler metal wire to increase the carbon content of the melted zone in specific proportions.